Inverters have been employed in many types of electrical equipments, as an efficient variable-speed control unit. Inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, followed by applying a voltage twice as high as the inverter output voltage at the maximum. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT, is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipments, such as inverter-related equipments, for example, high-speed switching devices, inverter motors, and transformers, insulated wires made of enameled wires are mainly used as magnet wires in the coils. Further, as described above, since a voltage almost twice as high as the inverter output voltage is applied in inverter-related equipments, it is required in insulated wires to have minimized partial discharge deterioration, which is attributable to inverter surge.
In general, partial discharge deterioration is a phenomenon in which an electrical insulating material undergoes, in a complicated manner, for example, molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge of the electrical insulating material, sputtering deterioration, thermal fusion or thermal decomposition deterioration caused by local temperature rise, and chemical deterioration caused by ozone generated due to discharge. For this reason, reduction in thickness, for example, is observed in the actual electrical-insulation materials, which have been deteriorated as a result of partial discharge.
In order to obtain an insulated wire in which no partial discharge is caused, i.e., an insulated wire having a high partial discharge inception voltage so as to prevent an insulated wire from the deterioration caused by such a partial discharge, such measures are studied as increasing the thickness of an insulating layer of the insulated wire, or using a resin having a low dielectric constant in the insulating layer.
However, when the thickness of the insulating layer is increased, the resultant insulated wire becomes thicker, and as a result, size enlargement of electrical equipments is brought about. This is retrograde to the demand in recent miniaturization of electrical equipments represented by motors and transformers. For example, specifically, it is no exaggeration to say that the performance of a rotator, such as a motor, is determined by how many electrical wires are held in a cross section of a stator slot. As a result, the ratio (space factor) of the sectional area of conductors to the sectional area of the stator slot, has been highly increased in recent years. Thus, if the thickness of the insulating layer is increased, the space factor is lowered, which is not preferable.
On the other hand, with respect to the dielectric constant of an insulating layer, most of resins that are generally used as a material for the insulating layer have a dielectric constant from 3 to 4, and thus there is no resin having a specifically low dielectric constant. Furthermore, in practice, a resin having a low dielectric constant cannot always be selected when other properties that are required for the insulating layer (heat resistance, solvent resistance, flexibility and the like) are taken into consideration.
As a means for decreasing the substantial dielectric constant of the insulating layer, such a measure is studied as foaming the insulating layer, and foamed electrical wires containing a conductor and a foamed insulating layer have been widely used as communication lines. Conventionally, foamed electrical wires such as those obtained by foaming an olefin-based resin such as polyethylene or a fluorine resin have been well-known. As examples of such foamed wires, foamed polyethylene insulating electrical wires are described in Patent Literatures 1 and 2, foamed fluorine resin insulating electrical wires are described in Patent Literatures 3 and 4, the both insulating electrical wires are described in Patent Literature 5, and a foamed polyolefin insulating electrical wire is described in Patent Literature 6. However, in such conventional foamed electrical wires, the dielectric breakdown voltage is decreased as the foaming magnification is increased.